1. Field of the Invention
This invention relates to a process for producing trichloroisocyanuric acid known as having a particularly high effective chlorine content among the chlorinated isocyanuric acids which are hydrolyzable in water to release active chlorine and are, therefore, used as disinfectants and bleaches. More particularly, it is an improvement in the process which produces trichloroisocyanuric acid by two stages of reaction between a mixture of cyanuric acid and an aqueous alkali solution and chlorine.
2. Description of the Prior Art
The production trichloroisocyanuric acid by two stages of reaction between a mixture of cyanuric acid and an aqueous alkali solution and chlorine is disclosed in Japanese Patent Publication No. 24902/1965, U.S. Pat. No. 3,534,033, the Japanese patent application laid open under No. 4484/1973, etc. Japanese Patent Publication No. 24902/1965 teaches that it is possible to achieve a high yield of trichloroisocyanuric acid continuously if the redox potential of the reaction product in a second reaction zone is measured by a platinum/NKCl/calomel electrode and maintained in the range of 1050 to 1150 mV, and if the supply of the reactants is controlled in accordance with the redox potential.
U.S. Pat. No. 3,534,033 and the Japanese patent application laid open under No. 4484/1973 teach that it is possible to produce trichloroisocyanuric acid efficiently if an alkali salt of cyanuric acid is reacted with chlorine in a first reaction zone to form a reaction product having a pH of at least 9 (U.S. Patent) or at least 12 (Japanese application), and if chlorine is added to the reaction product of the first reaction zone in a second reaction zone to reduce the formation of nitrogen trichloride so that the reaction product may have a pH of 1.5 to 4. In other words, they rely upon the pH of the reaction product for the control of the reaction in each of the first and second reaction zones.
Referring to the process as taught by Japanese Patent Publication No. 24902/1965, however, it is necessary to measure the redox potential of the reaction product in the second reaction zone in which the crystals of trichloroisocyanuric acid are formed. The adherence of crystals to the electrode and the failure of the electrode, which is usually a glass electrode, to remain stable for a long time make it difficult to maintain a really sharp, accurate and reliable redox potential for a long period of time. A similar problem is likely to arise from the process which relies upon the pH of the reaction product as taught by U.S. Pat. No. 3,534,033, etc.
The formation of nitrogen trichloride as a highly explosive by-product is a matter which is not ignorable when trichloroisocyanuric acid is produced on an industrial basis. It is known that the formation of nitrogen trichloride can be kept to a minimum if the reaction product of the first reaction zone is reacted with chlorine gas quickly under stirring in the second reaction zone in which its chlorination is completed. When the reactor is of the tank type, however, it is necessary to use a larger stirrer to ensure the circulation of the reaction product from the first reaction zone and promote the dissolution of chlorine gas on which the rate of the reaction depends. Nevertheless, there is no assurance whatsoever that the formation of nitrogen trichloride can be completely prevented, but there is every likelihood that nitrogen trichloride may be formed and cause an abnormal reaction. Therefore, the chlorinating step of the process for producing trichloroisocyanuric acid calls for an expersive apparatus which is not only made of a chlorine-resisting material, but can also withstand a certain level of pressure.